In the conventional pneumatic tires, particularly winter-seasoned tires such as studless tire or the like, there are proposed various methods for ensuring performances on ice or performances on snow.
As a way for increasing a friction coefficient on an ice-snow road surface, it is useful to microscopically form grooves by using a short fiber-containing foamed rubber as a tread rubber and breaking away the short fibers exposed on a surface of the tread accompanied with the wearing of the tread rubber during the running as described in JP-A-4-38207.
However, the microscopic grooves formed on the tread are liable to be crushed as a load applied to the tire becomes large, and hence the effect of increasing the friction coefficient on the ice-snow road surface can not be sufficiently obtained.
Also, it is necessary that the short fibers are embedded in the foamed rubber at a state of extending in a straight line and substantially in parallel to a worn face of the tread when the tread is worn by the running for rapidly breaking away the short fibers exposed on the worn face of the tread to microscopically form the grooves.
However, the short fibers are actually and frequently embedded in the foamed rubber at a state of being curled through heat shrinkage in the vulcanization or pushing the fibers into a groove portion of a mold part or a sipe portion to bend in the tread rubber. In this case, even if the tread worn during the running, the short fibers not extending substantially in parallel to the worn face of the tread can not easily be broken away from the foamed rubber, so that the microscopic grooves as originally intended can not be formed efficiently.
As another way for ensuring the performances on ice or performances on snow, it is useful to use a foamed rubber containing spherical closed cells in the tread as described in JP-A-62-283001.
That is, ice or snow on the ice-snow road surface is thawed to water by friction heat when the tire tread contacts with the ice-snow road surface and this water forms a water membrane between the tread and the ice-snow road surface, and the performances on ice or performances on snow are deteriorated by this water membrane. In case of the tire using the above foamed rubber, the water membrane can be removed by the action of irregularities on the tread formed by the closed cells, and hence it is attempted to improve the performances on ice or the performances on snow.
However, the tire described in JP-A-62-283001 does not develop the sufficient water-removing effect because the irregularities formed on the tread by the closed cell are very fine.
For this end, the applicant has made studies for enhancing the water removing effect on the ice-snow road surface and found that the water removing effect on the ice-snow road surface is conspicuous in pneumatic tires provided with a tread having not only the spherical closed cells but also many continuous closed cells covered with a protection layer of a resin, which has been proposed by an Application No. JP9700873 of an international application under PCT (international publication No. 9734776).
In such a tire, it is considered that drainage paths can be particularly formed by the continuous closed cells, whereby the water removing effect is enhanced.
However, the applicant has made further studies in order to more enhance the water removing effect on the ice-snow road surface and revealed that there is room for improvement because it is hardly said that the sufficient drainage paths are formed in the tire as the length of the continuous closed cell is shorter than an arranging pitch of sipe or groove disposed on the tread.